In recent years, a compact and low power consumption wireless communication system such as WPAN (Wireless Personal Area Network) or a sensor network is attracting attention. There is a similar system which uses active RF tags that autonomously transmits wireless signals.
For example, a communication terminal used in the above-described systems has such uses as: reading a meter of gas, electricity, water or the like; outdoors monitoring of an air pollution concentration, a pollen count, a temperature or the like; a home security system like a fire-alarm box, an entrance detection sensor or the like; and location management of a child, an elderly person or the like. Accordingly, the communication terminal is required to be compact, and, though not required to have a high performance relating to a transmission rate or in a communication distance, required to have such an ultra low power consumption that allows the communication terminal to be driven by a small battery for several years.
The above-described system generally has, for example, a relatively low transmission rate of about several kbps to several hundreds kbps, and a short wireless signal transmission range of about several meters to several tens of meters. Accordingly, an output power can be smaller, so that power consumption can be kept low. However, there is a limit in allowing the above-described communication terminal to be driven by a small battery for a long period of time only by designing the transmission rate to be low and the transmission range to be short as described above. Alternatively, various arts to reduce power consumption are suggested for further downsizing and further lengthening of operating life.
As a conventional art to reduce a power consumption of the above-described communication terminal, an art is disclosed, for example, in which a period of time for transmitting and receiving data in a low traffic network such as a sensor network and the like is preliminarily specified, and a supply of power to a unit related to a wireless communication function is stopped except for at the above-described period of time (see Patent Literature 1 and the “IEEE 802.15.4” standard). In such a conventional art, in order to adjust the period of time for transmitting and receiving data between apparatuses, a synchronous mode referred to as a beacon mode is used.
FIG. 14 is a diagram illustrating an outline of the beacon mode in the wireless communication system described in Patent Literature 1.
In FIG. 14, time advances from left to right, and an active interval and an inactive interval are set to be alternately repeated. A base station transmits a wireless packet referred to as a beacon at the beginning of the active interval which has been set by the base station, and thereafter, transmits and receives data only in the active interval but not in the inactive interval which has been set by the base station. Here, the beacon is a wireless packet in which synchronization information necessary for synchronizing the respective apparatuses is written in a data field, and the synchronization information is, for example, information indicating a beacon time period T_b until subsequent beacons are to be transmitted. When receiving a beacon transmitted from the base station, the communication terminal recognizes that there is an active interval with the beacon at the beginning. Further, after performing necessary transmission and reception during the active interval, the communication terminal recognizes a transmission timing of the subsequent beacon and a subsequent inactive interval based on information included in the received beacon and indicating the beacon time period T_b, thereby stopping a supply of power to the units relating to the wireless communication function.
Citation List
[Patent Literature]
[PTL 1] (PCT) International Publication WO2008/023638